This invention relates generally to methods and apparatus for well logging and more specifically relates to methods and apparatus for injecting a tracer element into the fluid flow within a well to assist in the determination of parameters concerning the movement of such fluid flow.
Wells drilled for use in the production of oil or gas may be utilized either as producing wells, from which the oil or gas is obtained, or as injection wells, through which fluids, such as polymer compounds or salt water, are forced into the earth formations surrounding the well to force the oil or gas through the formations to thereby stimulate production in nearby wells. Wells may often be drilled as producing wells and subsequently converted to use as injection wells when such change in usage becomes economically preferable.
In a producing well, the oil or gas may enter the well from one or more depth horizons or zones within the earth formations surrounding the well. When the oil or gas is being produced from more than one zone it is desirable to determine how much of the fluid is being produced from each zone. This determination may be made by measuring the velocity of the well fluid at various depth locations within the well, preferably at locations between the known producing zones. From such fluid velocity measurements and knowledge of both the diameter of the well at each depth location and of the diameter of the logging instrument used to measure the fluid velocity, the volumetric flow rate of the well fluid at each depth location may be determined, thereby facilitating a determination of the proportional contribution of each producing zone to the total flow rate of the well.
An analogous but reverse situation is presented in the case of injection wells. When the polymers, salt water, or other fluids are being injected, or pumped, into the well, it is desirable to determine the proportion of the injection fluid which is entering each zone within the well so as to determine if the desired zones are receiving the injection fluid. By measuring the velocity of the injection fluid and by determining the fluid flow rate therefrom at various depth locations within the well, the proportion of the injection fluid entering each zone may be determined.
One means which has been utilized by the oil and gas industry to make such fluid velocity measurements, and in turn such flow rate determinations, has been to inject a tracer element, such as a radioactive isotope, into a well fluid flow and to measure the time required for the tracer element to traverse a known distance within the well. In the case of a radioactive tracer this may be accomplished by injecting the tracer element from a logging instrument upstream in the fluid flow from a pair of suitable radiation detectors, such as geiger counter tubes or scintillation counters, spaced a known fixed distance apart along the longitudinal axis of the logging instrument. The time between the passing by the tracer element of the first detector to the passing of the second detector may then be utilized to determine the velocity of the well fluid. This tracer injection method of flow rate determination is particularly useful in production wells with low flow rates and in injection wells where polymers are being injected because low flow rates and/or relatively high viscosity fluids, such as polymers, often cause other types of flow meters to yield less than optimal data in flow rate measurements.
The injection of a tracer element into a well fluid flow is also used by the oil and gas industry in logging operations to determine the location of fluid flow in wells rather than the rate of the fluid flow. This type of logging operation also aids in determining zones into which fluid flows in injection wells and may additionally be utilized in both production and injection wells to determine and locate mechanical breakdowns within the well such as holes or leaks in the well casing or channeling within the cement or earth formations surrounding the casing. In this type of logging operation, the tracer element, again preferably a radioactive tracer, is injected into the well fluid flow and one or more suitable detectors of the tracer are traversed through at least a portion of the well to determine the location of the tracer. Preferably, a plurality of such traversals are made over selected increments of time so as to monitor the travel of the tracer to aid in the determination of the presence of any channelling as described earlier herein.
The tracer element may be injected into the well fluid in the form of a globule or blob or it may be preferable to inject the tracer element into the well fluid such that the tracer element disperses in a cloud-like diffusion within the well fluid. Regardless of the form in which the tracer element is to be injected into the well fluid, in both types of logging operations described earlier herein it is desirable to inject the tracer element such that the tracer element is moved and carried by the well fluid in a manner generally indicative of the movement of the well fluid. Such manner of injection is complicated by the fact that all of the fluid flowing within the well does not travel at a uniform velocity. The general tendency is that the fluid near the sidewalls or casing of the well and fluid adjacent a logging instrument placed within the well will flow at a significantly lower velocity than will the fluid flowing near the center of the well bore. Because this fluid flowing near the center of the well bore typically exhibits the maximum velocity at any one depth horizon within the well and, correspondingly, the maximum volumetric flow rate at such depth horizon, it is optimal to inject the tracer element into the fluid flowing proximate the effective center of the well bore.
Apparatus commonly used in the oil and gas industry for injecting tracer elements into the well fluid have typically injected the tracer element in a generally lateral direction relative to the longitudinal axis of the well bore, such injection typically being accomplished by forcing the tracer element through one or more jets or nozzles located on the periphery of a logging instrument. Further, the logging instrument in such injection operations has typically been centralized along the longitudinal axis of the well bore. When the logging instrument is centralized within the well there is an annulus between the outer diameter of the logging instrument and the inner diameter of the well bore. The maximum flow rate past the logging instrument will typically be found proximate an area located along the radial center of the annulus, with the lowest flow rates typically being found in areas adjacent the exterior of the logging instrument and adjacent the inner perimeter of the well. Dependent upon the outer diameter of the logging instrument and the inner diameter of the well or casing, the annulus may range from approximately one inch to over a foot in radial dimension. Additionally, because the inner diameter of the well bore or casing may differ over the depth of the well, annuli of several sizes may be encountered within a single well. It can, therefore, be appreciated that with the described commonly used logging instruments which eject the tracer element laterally into the well fluid, it can be virtually impossible to continually accurately place the tracer element into the maximum flow of the well fluid at the center of the described annulus. It can also be appreciated that if such tracer element is not placed proximate the cross-sectional center of the area of fluid flow, the uneven velocities within the fluid flow may lead to less than optimal data regarding the flow rates and areas of penetration of the fluid flow.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a method and apparatus whereby tracer elements may be accurately and repeatedly placed in a desired proportional location across the fluid flow column within a well and by which characteristics of the movement of such fluid may be determined.